The invention relates to a method of making a metal oxide-carbon nanocomposite material, and more particularly, to a method of making a metal oxide-carbon nanocomposite material from a layered double hydroxide or hydrotalcite-like material for use in both anion and cation sorption applications.
Hydrotalcite is a naturally occurring anionic clay material in which carbonate ions are located between positively charged sheets of metal hydroxides, having the idealized unit cell formula [Mg6Al2(OH)16(CO3).4H2O]. Synthetic hydrotalcite-like materials, known as layered double hydroxide (LDH) materials, are useful as contact solids in catalytic, oxidative and absorbent processes. LDHs are a group of anionic materials that have positively charged sheets of metal hydroxides, between which are located anions and, in general, some water molecules. Most common LDHs are based on double hydroxides of such main group metals as Mg, and Al and transition metals such as Ni, Co, Cr, Zn and Fe etc. These materials have structures similar to brucite [Mg(OH)2] in which the magnesium are octahedrally surrounded by hydroxyl groups with the resulting octahedra sharing edges to form infinite sheets. In the LDHs, some of the magnesium is isomorphously replaced by a trivalent ion, such as Al3+. The Mg2+, Al3+, and OH− layers are then positively charged, necessitating charge balancing by insertion of anions between the layers. Various other divalent and trivalent ions can be substituted for Mg and Al. In addition, the anion, which is carbonate in hydrotalcite, can be varied in synthesis by a large number of simple anions such as NO3−, Cl−, OH−, and SO42−. These LDHs, based on their structure, fall into the Hydrotalcite-Manasseite and Pyroaurite-Sjogrenite groups, where brucite-like layers carrying net positive charge alternate with interlayer spaces containing carbonate or other anionic groups and water molecules.
Hydrocalumite and related synthetic compounds also have a layered structure in which positively charged metal hydroxide layers alternate with the interlayers containing anions and water. The hydroxide layers contain specific combinations of metal ions derived from on one hand divalent calcium cations and on the other from trivalent cations of metals such as iron, or more particularly, aluminum. The interlayers contain anions such as OH−, SO42−, Cl−, NO3− and, in particular, CO32−. The general formula for the group is [Ca2M3+(OH)6]X.yH2O, where M3+ is a tripositive ion and typically Al3+, X is a singly charged anion or equal amounts of more highly charged ones, and y is between 2 and 6. As in the Pyroaurite-Sjogrenite group, principal layers alternate with inter-layers, the principal layers having the composition [Ca2M3+(OH)6]+ and the interlayers consisting of water molecules and anion X. However, because of the difference in size between the Ca2+ and Al3+, the M2+:M3+ ratio is fixed at 2:1 and their arrangement is ordered.
The syntheses of LDHs are generally simple, and the so-called “precipitation method” is most popular. If a carbonate-containing product is desired, then the aqueous solution of magnesium and aluminum salts, such as nitrate or chloride salts, is added to an aqueous solution of sodium hydroxide-carbonate with good mixing at room temperature. The resulting amorphous precipitate is then heated for several hours at 60° C.-200° C. to obtain a crystalline material. Washing and drying complete the synthesis in quantitative yield. By employing this precipitation method, replacement of all or part of Mg2+ with MII ions such as Ca2+, Zn2+, and Cu2+, or replacement of Al3+ with other MIII ions such as Fe3+ and Cr3+, is also possible.
One aspect of the synthesis of these materials is the variation of the nature of the interstitial anion. The preparation of hydrotalcite-like materials with anions other than carbonate in pure form requires special procedures, because LDH incorporates carbonate in preference to other anions. Most of the time, the smaller anions are introduced to the LDH structure via the precipitation method by using the desired anion solutions instead of carbonate. However, in these methods, the synthesis has to be carried out in a controlled atmosphere to prevent carbonate contamination from the atmospheric carbon dioxide.
Pinnavaia et al. (U.S. Pat. No. 5,114,898) describe LDH sorbents for the removal of SOx from gas streams, where the interlayer anion forms a volatile gas at elevated temperatures and where a metal cation is impregnated as a salt to provide oxidation of sulfur dioxide to sulfur trioxide. Pinnavaia et al. (U.S. Pat. No. 5,069,203) disclose LDH materials interlayered by polyoxometalate ions. Pinnavaia et al. (U.S. Pat. No. 5,463,042) disclose LDH materials interlayered with a metal complex of a polyaryl compound, such as a porphyrin or phthalocyanine. Albers et al. (U.S. Pat. No. 6,156,696) describe LDH materials that incorporate organic acid anionic species to result in crystalline sheet contact materials having increased sorption of SOx. Shutz et al. (U.S. Pat. No. 5,399,329) disclose the synthesis of hydrotalcite-like materials using magnesium and aluminum compounds as the metal ions in the positively charged sheets and using mono carboxylic anions to produce intercalary materials.